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<div class="section" id="functional-programming">
<span id="index-0"></span><h1>Functional Programming<a class="headerlink" href="#functional-programming" title="Permalink to this headline">¶</a></h1>
<p>In previous chapters we learned about Functions and Recursion.</p>
<p>In this chapter we are going to learn about more Functional Programming (FP) concepts like</p>
<ul class="simple">
<li><p>Pure Functions</p></li>
<li><p>First-class functions</p></li>
<li><p>Higher-order functions</p></li>
<li><p>Anonymous and nested functions.</p></li>
<li><p>Equality of functions</p></li>
</ul>
<div class="section" id="pure-functions">
<span id="index-1"></span><h2>Pure Functions<a class="headerlink" href="#pure-functions" title="Permalink to this headline">¶</a></h2>
<p>We can create pure functions (functions that doesn’t change the state) by the help of the assignment operator to
copy variables (Lists & Objects) by value to create new variables instead of working on the original data that are passed
to the function by reference.</p>
<p>Example:</p>
<div class="highlight-ring notranslate"><div class="highlight"><pre><span></span><span class="k">Func</span> <span class="n">Main</span>
<span class="n">aList</span> <span class="o">=</span> <span class="o">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">5</span><span class="o">]</span>
<span class="n">aList2</span> <span class="o">=</span> <span class="n">square</span><span class="p">(</span><span class="n">aList</span><span class="p">)</span>
<span class="k">see</span> <span class="s">"aList"</span> <span class="o">+</span> <span class="n">nl</span>
<span class="k">see</span> <span class="n">aList</span>
<span class="k">see</span> <span class="s">"aList2"</span> <span class="o">+</span> <span class="n">nl</span>
<span class="k">see</span> <span class="n">aList2</span>
<span class="k">Func</span> <span class="n">Square</span> <span class="n">aPara</span>
<span class="n">a1</span> <span class="o">=</span> <span class="n">aPara</span> <span class="c"># copy the list</span>
<span class="k">for</span> <span class="n">x</span> <span class="k">in</span> <span class="n">a1</span>
<span class="n">x</span> <span class="o">*=</span> <span class="n">x</span>
<span class="k">next</span>
<span class="k">return</span> <span class="n">a1</span> <span class="c"># return new list</span>
</pre></div>
</div>
<p>Output:</p>
<div class="highlight-ring notranslate"><div class="highlight"><pre><span></span><span class="n">aList</span>
<span class="mi">1</span>
<span class="mi">2</span>
<span class="mi">3</span>
<span class="mi">4</span>
<span class="mi">5</span>
<span class="n">aList2</span>
<span class="mi">1</span>
<span class="mi">4</span>
<span class="mi">9</span>
<span class="mi">16</span>
<span class="mi">25</span>
</pre></div>
</div>
</div>
<div class="section" id="first-class-functions">
<span id="index-2"></span><h2>First-class Functions<a class="headerlink" href="#first-class-functions" title="Permalink to this headline">¶</a></h2>
<p>Functions inside the Ring programming language are first-class citizens, you can pass functions as parameters, return them as
value or store them in variables.</p>
<p>We can pass/return the function by typing the function name as literal like “FunctionName” or :FunctionName for example.</p>
<p>We can pass/return functions using the variable that contains the function name.</p>
<p>We can call function from variables contains the function name using the Call command</p>
<p>Syntax:</p>
<div class="highlight-ring notranslate"><div class="highlight"><pre><span></span><span class="k">Call</span> <span class="n">Variable</span><span class="p">(</span><span class="o">[</span><span class="n">Parameters</span><span class="o">]</span><span class="p">)</span>
</pre></div>
</div>
<p>Example:</p>
<div class="highlight-ring notranslate"><div class="highlight"><pre><span></span><span class="k">Func</span> <span class="n">Main</span>
<span class="k">see</span> <span class="s">"before test2()"</span> <span class="o">+</span> <span class="n">nl</span>
<span class="n">f</span> <span class="o">=</span> <span class="n">Test2</span><span class="p">(:</span><span class="n">Test</span><span class="p">)</span>
<span class="k">see</span> <span class="s">"after test2()"</span> <span class="o">+</span> <span class="n">nl</span>
<span class="k">call</span> <span class="n">f</span><span class="p">()</span>
<span class="k">Func</span> <span class="n">Test</span>
<span class="k">see</span> <span class="s">"Message from test!"</span> <span class="o">+</span> <span class="n">nl</span>
<span class="k">Func</span> <span class="n">Test2</span> <span class="n">f1</span>
<span class="k">call</span> <span class="n">f1</span><span class="p">()</span>
<span class="k">See</span> <span class="s">"Message from test2!"</span> <span class="o">+</span> <span class="n">nl</span>
<span class="k">return</span> <span class="n">f1</span>
</pre></div>
</div>
<p>Output:</p>
<div class="highlight-ring notranslate"><div class="highlight"><pre><span></span><span class="n">before</span> <span class="n">test2</span><span class="p">()</span>
<span class="n">Message</span> <span class="k">from</span> <span class="n">test</span><span class="o">!</span>
<span class="n">Message</span> <span class="k">from</span> <span class="n">test2</span><span class="o">!</span>
<span class="n">after</span> <span class="n">test2</span><span class="p">()</span>
<span class="n">Message</span> <span class="k">from</span> <span class="n">test</span><span class="o">!</span>
</pre></div>
</div>
</div>
<div class="section" id="higher-order-functions">
<span id="index-3"></span><h2>Higher-order Functions<a class="headerlink" href="#higher-order-functions" title="Permalink to this headline">¶</a></h2>
<p>Higher-order functions are the functions that takes other functions as parameters.</p>
<p>Example:</p>
<div class="highlight-ring notranslate"><div class="highlight"><pre><span></span><span class="k">Func</span> <span class="n">Main</span>
<span class="n">times</span><span class="p">(</span><span class="mi">5</span><span class="p">,:</span><span class="n">test</span><span class="p">)</span>
<span class="k">Func</span> <span class="n">Test</span>
<span class="k">see</span> <span class="s">"Message from the test function!"</span> <span class="o">+</span> <span class="n">nl</span>
<span class="k">Func</span> <span class="n">Times</span> <span class="n">nCount</span><span class="p">,</span><span class="n">F</span>
<span class="k">for</span> <span class="n">x</span> <span class="o">=</span> <span class="mi">1</span> <span class="k">to</span> <span class="n">nCount</span>
<span class="k">Call</span> <span class="n">F</span><span class="p">()</span>
<span class="k">next</span>
</pre></div>
</div>
<p>Output:</p>
<div class="highlight-ring notranslate"><div class="highlight"><pre><span></span><span class="n">Message</span> <span class="k">from</span> <span class="n">the</span> <span class="n">test</span> <span class="n">function</span><span class="o">!</span>
<span class="n">Message</span> <span class="k">from</span> <span class="n">the</span> <span class="n">test</span> <span class="n">function</span><span class="o">!</span>
<span class="n">Message</span> <span class="k">from</span> <span class="n">the</span> <span class="n">test</span> <span class="n">function</span><span class="o">!</span>
<span class="n">Message</span> <span class="k">from</span> <span class="n">the</span> <span class="n">test</span> <span class="n">function</span><span class="o">!</span>
<span class="n">Message</span> <span class="k">from</span> <span class="n">the</span> <span class="n">test</span> <span class="n">function</span><span class="o">!</span>
</pre></div>
</div>
</div>
<div class="section" id="anonymous-and-nested-functions">
<span id="index-4"></span><h2>Anonymous and Nested Functions<a class="headerlink" href="#anonymous-and-nested-functions" title="Permalink to this headline">¶</a></h2>
<p>Anonymous Functions are functions without names that can be passed as parameters to other functions or stored in variables.</p>
<p>Syntax:</p>
<div class="highlight-ring notranslate"><div class="highlight"><pre><span></span><span class="k">Func</span> <span class="o">[</span><span class="n">Parameters</span><span class="o">]</span> <span class="p">{</span> <span class="o">[</span><span class="n">statements</span><span class="o">]</span> <span class="p">}</span>
</pre></div>
</div>
<p>Example:</p>
<div class="highlight-ring notranslate"><div class="highlight"><pre><span></span><span class="n">test</span><span class="p">(</span> <span class="k">func</span> <span class="n">x</span><span class="p">,</span><span class="n">y</span> <span class="p">{</span>
<span class="k">see</span> <span class="s">"hello"</span> <span class="o">+</span> <span class="n">nl</span>
<span class="k">see</span> <span class="s">"Sum : "</span> <span class="o">+</span> <span class="p">(</span><span class="n">x</span><span class="o">+</span><span class="n">y</span><span class="p">)</span> <span class="o">+</span> <span class="n">nl</span>
<span class="p">}</span> <span class="p">)</span>
<span class="k">new</span> <span class="n">great</span> <span class="p">{</span> <span class="n">f1</span><span class="p">()</span> <span class="p">}</span>
<span class="n">times</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="k">func</span> <span class="p">{</span> <span class="k">see</span> <span class="s">"hello world"</span> <span class="o">+</span> <span class="n">nl</span> <span class="p">}</span> <span class="p">)</span>
<span class="k">func</span> <span class="n">test</span> <span class="n">x</span>
<span class="k">call</span> <span class="n">x</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span><span class="mi">3</span><span class="p">)</span>
<span class="k">see</span> <span class="s">"wow!"</span> <span class="o">+</span> <span class="n">nl</span>
<span class="k">func</span> <span class="n">times</span> <span class="n">n</span><span class="p">,</span><span class="n">x</span>
<span class="k">for</span> <span class="n">t</span><span class="o">=</span><span class="mi">1</span> <span class="k">to</span> <span class="n">n</span>
<span class="k">call</span> <span class="n">x</span><span class="p">()</span>
<span class="k">next</span>
<span class="k">Class</span> <span class="n">great</span>
<span class="k">func</span> <span class="n">f1</span>
<span class="n">f2</span><span class="p">(</span> <span class="k">func</span> <span class="p">{</span> <span class="k">see</span> <span class="s">"Message from f1"</span> <span class="o">+</span> <span class="n">nl</span> <span class="p">}</span> <span class="p">)</span>
<span class="k">func</span> <span class="n">f2</span> <span class="n">x</span>
<span class="k">call</span> <span class="n">x</span><span class="p">()</span>
</pre></div>
</div>
<p>Output:</p>
<div class="highlight-ring notranslate"><div class="highlight"><pre><span></span><span class="n">hello</span>
<span class="n">Sum</span> <span class="p">:</span> <span class="mi">6</span>
<span class="n">wow</span><span class="o">!</span>
<span class="n">Message</span> <span class="k">from</span> <span class="n">f1</span>
<span class="n">hello</span> <span class="n">world</span>
<span class="n">hello</span> <span class="n">world</span>
<span class="n">hello</span> <span class="n">world</span>
</pre></div>
</div>
<p>Example:</p>
<div class="highlight-ring notranslate"><div class="highlight"><pre><span></span><span class="k">Func</span> <span class="n">Main</span>
<span class="n">aList</span> <span class="o">=</span> <span class="o">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="o">]</span>
<span class="n">Map</span> <span class="p">(</span><span class="n">aList</span> <span class="p">,</span> <span class="k">func</span> <span class="n">x</span> <span class="p">{</span>
<span class="k">return</span> <span class="n">x</span><span class="o">*</span><span class="n">x</span>
<span class="p">}</span> <span class="p">)</span>
<span class="k">see</span> <span class="n">aList</span>
<span class="n">aList</span> <span class="o">=</span> <span class="o">[</span><span class="mi">4</span><span class="p">,</span><span class="mi">9</span><span class="p">,</span><span class="mi">14</span><span class="p">,</span><span class="mi">25</span><span class="o">]</span>
<span class="n">Map</span><span class="p">(</span><span class="n">aList</span><span class="p">,</span> <span class="p">:</span><span class="n">myfilter</span> <span class="p">)</span>
<span class="k">see</span> <span class="n">aList</span>
<span class="n">aList</span> <span class="o">=</span> <span class="o">[</span><span class="mi">11</span><span class="p">,</span><span class="mi">12</span><span class="p">,</span><span class="mi">13</span><span class="p">,</span><span class="mi">14</span><span class="o">]</span>
<span class="n">Map</span> <span class="p">(</span><span class="n">aList</span> <span class="p">,</span> <span class="k">func</span> <span class="n">x</span> <span class="p">{</span>
<span class="k">if</span> <span class="n">x</span><span class="o">%</span><span class="mi">2</span><span class="o">=</span><span class="mi">0</span>
<span class="k">return</span> <span class="s">"even"</span>
<span class="k">else</span>
<span class="k">return</span> <span class="s">"odd"</span>
<span class="k">ok</span>
<span class="p">})</span>
<span class="k">see</span> <span class="n">aList</span>
<span class="k">Func</span> <span class="n">myfilter</span> <span class="n">x</span>
<span class="k">if</span> <span class="n">x</span> <span class="o">=</span> <span class="mi">9</span>
<span class="k">return</span> <span class="s">"True"</span>
<span class="k">else</span>
<span class="k">return</span> <span class="s">"False"</span>
<span class="k">ok</span>
<span class="k">Func</span> <span class="n">Map</span> <span class="n">aList</span><span class="p">,</span><span class="n">cFunc</span>
<span class="k">for</span> <span class="n">x</span> <span class="k">in</span> <span class="n">aList</span>
<span class="n">x</span> <span class="o">=</span> <span class="k">call</span> <span class="n">cFunc</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="k">next</span>
</pre></div>
</div>
<p>Output:</p>
<div class="highlight-ring notranslate"><div class="highlight"><pre><span></span><span class="mi">1</span>
<span class="mi">4</span>
<span class="mi">9</span>
<span class="mi">16</span>
<span class="kp">False</span>
<span class="kp">True</span>
<span class="kp">False</span>
<span class="kp">False</span>
<span class="n">odd</span>
<span class="n">even</span>
<span class="n">odd</span>
<span class="n">even</span>
</pre></div>
</div>
</div>
<div class="section" id="equality-of-functions">
<span id="index-5"></span><h2>Equality of functions<a class="headerlink" href="#equality-of-functions" title="Permalink to this headline">¶</a></h2>
<p>We can test if function = function or not using the ‘=’ or ‘!=’ operators</p>
<p>Example:</p>
<div class="highlight-ring notranslate"><div class="highlight"><pre><span></span><span class="n">f1</span> <span class="o">=</span> <span class="k">func</span> <span class="p">{</span> <span class="k">see</span> <span class="s">"hello"</span> <span class="o">+</span> <span class="n">nl</span> <span class="p">}</span>
<span class="n">f2</span> <span class="o">=</span> <span class="k">func</span> <span class="p">{</span> <span class="k">see</span> <span class="s">"how are you?"</span> <span class="o">+</span> <span class="n">nl</span> <span class="p">}</span>
<span class="n">f3</span> <span class="o">=</span> <span class="n">f1</span>
<span class="k">call</span> <span class="n">f1</span><span class="p">()</span>
<span class="k">call</span> <span class="n">f2</span><span class="p">()</span>
<span class="k">call</span> <span class="n">f3</span><span class="p">()</span>
<span class="k">see</span> <span class="p">(</span><span class="n">f1</span> <span class="o">=</span> <span class="n">f2</span><span class="p">)</span> <span class="o">+</span> <span class="n">nl</span>
<span class="k">see</span> <span class="p">(</span><span class="n">f2</span> <span class="o">=</span> <span class="n">f3</span><span class="p">)</span> <span class="o">+</span> <span class="n">nl</span>
<span class="k">see</span> <span class="p">(</span><span class="n">f1</span> <span class="o">=</span> <span class="n">f3</span><span class="p">)</span> <span class="o">+</span> <span class="n">nl</span>
</pre></div>
</div>
<p>Output:</p>
<div class="highlight-ring notranslate"><div class="highlight"><pre><span></span><span class="n">hello</span>
<span class="n">how</span> <span class="n">are</span> <span class="n">you</span><span class="o">?</span>
<span class="n">hello</span>
<span class="mi">0</span>
<span class="mi">0</span>
<span class="mi">1</span>
</pre></div>
</div>
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